Rainforests are dense, wet forests that receive high year-round rainfall — typically over 2,000 mm annually in the tropics — and they are Earth's richest ecosystems on land. Although tropical rainforests cover only around 6% of the planet's land surface, they are home to roughly half of all known terrestrial plant and animal species and play a major role in regulating the global climate and water cycle.
The nickname "lungs of the Earth" is only half right — and the real story is more interesting. Mature rainforests consume most of the oxygen they produce, but what they genuinely do for the planet is arguably more important than making oxygen: they store vast quantities of carbon, recycle water on a continental scale, and shelter an irreplaceable library of life. This guide covers where rainforests grow, how they are built, why they matter, and what is happening to them.
What Counts as a Rainforest
A rainforest is defined by abundant, reliable moisture — enough that the forest never experiences a true dry season severe enough to halt growth. Two broad kinds exist:
- Tropical rainforests straddle the equator between roughly 10°N and 10°S, where temperatures hover around 25–28°C all year and rain falls in nearly every month. These are the classic jungles of the Amazon, Congo, and Southeast Asia.
- Temperate rainforests grow in cool, wet coastal regions — the Pacific Northwest of North America (including the Hoh Rainforest), southern Chile, New Zealand, and pockets of Australia and East Asia. They are dominated by giant conifers or southern beeches rather than broadleaf trees.
This article focuses mainly on tropical rainforests, which hold the overwhelming share of the world's biodiversity. Conditions inside them are remarkably stable: little seasonal temperature change, humidity typically over 75%, and rain on most days — a hothouse steadiness that has allowed evolution to run uninterrupted experiments for tens of millions of years.
The World's Three Great Rainforest Realms
- The Amazon — by far the largest, covering roughly 5.5 million km² across nine South American countries, about 60% of it in Brazil. It contains an estimated 390 billion trees and produces so much moisture that it generates much of its own rainfall. Start with our Amazon rainforest geography guide.
- The Congo Basin — Africa's great forest, the world's second largest, spanning six countries and famous for gorillas, forest elephants, and okapi. See our Congo rainforest guide, or compare the two giants in our Amazon vs. Congo comparison.
- Southeast Asia — the ancient forests of Borneo, Sumatra, and New Guinea, home to orangutans, tigers, and the giant rafflesia flower. Australia's Daintree is considered one of the oldest surviving rainforests on Earth, with a lineage stretching back well over 100 million years.
Life in Layers: How a Rainforest Is Built
A rainforest is a vertical city with distinct floors:
- Emergent layer (40–60+ m): Scattered giant trees tower above everything, braving sun, wind, and eagles.
- Canopy (25–40 m): The forest's roof — a continuous green ceiling where most rainforest species actually live, from monkeys and sloths to countless insects, orchids, and bromeliads.
- Understory: A dim world of saplings, shrubs, and vines receiving only a small fraction of the sunlight, patrolled by jaguars and leopards.
- Forest floor: Nearly dark, surprisingly open, and dominated by decomposition — fungi and insects break down fallen material so fast that most nutrients are held in the living forest rather than the thin soil.
That last fact explains a paradox: rainforest soils are often poor. Clear the forest, and the land's fertility frequently washes away within a few harvests.
The scale of life in these layers defies intuition. A single hectare of Amazon forest can contain more tree species than all of Europe; one rainforest tree can host hundreds of insect species; and scientists estimate that a large share of rainforest species — especially insects, fungi, and canopy dwellers — have never been formally described. Every survey of a new patch of canopy tends to turn up organisms unknown to science.
Why "Lungs of the Earth" Is Only Half True
Rainforest plants do release enormous amounts of oxygen through photosynthesis — the Amazon alone is often credited with several percent of the oxygen produced on land. But a mature forest also respires: its animals, microbes, and the plants themselves consume most of that oxygen again, so the net contribution to the air we breathe is modest (most of the atmosphere's free oxygen was banked by ocean plankton over billions of years). The rainforest's real planetary services are different:
- Carbon storage: Tropical forests hold hundreds of billions of tonnes of carbon in their wood and soils. Burning or clearing them releases it, accelerating climate change.
- Water recycling: Amazon trees transpire so much water vapor that they create "flying rivers" of moisture that water farmland far beyond the forest — a single large tree can release hundreds of liters per day.
- Biodiversity banking: Roughly half of terrestrial species live in tropical forests, including wild relatives of major food crops and the sources of a large share of modern medicines.
Rainforests vs. Coral Reefs: Two Biodiversity Superpowers Compared
Rainforests are often compared with coral reefs — the "rainforests of the sea" — and the parallels are striking. Both concentrate staggering biodiversity into a small footprint: forests on ~6% of land, reefs on well under 1% of the seafloor. Both are built by living things (trees; coral polyps) whose structures house everything else. Both thrive in nutrient-poor settings by recycling relentlessly. And both face the same pairing of local threats (clearing; destructive fishing) and global ones (warming climate; warming, acidifying seas). The key difference is speed of recovery: a cleared forest can regrow substantially in decades, while a reef's limestone framework takes centuries to rebuild — though an old-growth forest's full richness likewise needs centuries to return.
Deforestation: The Threat and the Response
The world loses several million hectares of tropical primary forest each year — driven chiefly by cattle ranching and soy in the Amazon, oil palm in Southeast Asia, and small-scale agriculture and logging in the Congo Basin. The consequences compound: lost species, released carbon, disrupted rainfall, and displaced forest peoples who have stewarded these ecosystems for millennia — the Amazon alone is home to hundreds of Indigenous groups. See where losses are concentrated in our guide to global deforestation hotspots.
There is genuine progress to build on: deforestation rates fall when protections are enforced, Indigenous-managed territories consistently show lower forest loss, and initiatives from debt-for-nature swaps to zero-deforestation supply chains are expanding. Which countries still hold the most forest? Our list of countries with the most rainforest has the ranking.
Scientists also warn of tipping points. The Amazon depends on moisture its own trees recycle; clear too much of it — estimates often cited fall somewhere beyond 20–25% loss — and parts of the forest could shift toward drier savanna in a self-reinforcing spiral. That risk, more than any single year's clearing statistics, is why the world's remaining large, intact forests are treated as global priorities rather than local resources.
Key Facts at a Glance
- Tropical rainforests cover about 6% of land but host roughly 50% of terrestrial species.
- The Amazon is the largest rainforest (~5.5 million km²), followed by the Congo Basin and Southeast Asia's forests.
- Rainforests are organized in four layers, with most life in the canopy.
- Their greatest planetary roles are carbon storage and continental-scale water recycling — not net oxygen supply.
- Temperate rainforests exist too, from the Pacific Northwest to Chile and New Zealand.
- Rainforest soils are typically poor — the nutrients live in the forest itself, which is why cleared land degrades quickly.
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